a) Field of the Invention
The present invention is in the field of continuous temperature measurement. In particular, the present invention relates to the continuous temperature measurement of molten metal during the continuous casting thereof. Specifically, it is critical to control this temperature in order to ensure optimum quality of the cast metal.
b) Description of the Related Art
To this end, a device comprising a sleeve made of refractory material dipped into the bath of molten metal and coupled to temperature sensors is very widespread. Use is mainly made of two types of sensors, namely sensors that use type B thermocouples or thermocouples made of an alloy of noble metals, and infrared optical sensors. The drawback with the former is their premature ageing due to the extreme use conditions. Frequent replacement of the sensors has to be carried out, and this represents a non-negligible cost. Infrared optical sensors, also known as optical pyrometers, have, in contrast to thermocouples, a much longer service life while providing temperature measurements that are precise and stable over time as long as the measuring zone remains centred in the interior of the sleeve.
The International Application WO-A2-03/029771 discloses an assembly of an optical pyrometer and a sleeve made of refractory material which is dipped into a bath of molten metal in order to measure the temperature thereof. The pyrometer and the protective sleeve are secured so as to keep the measuring zone centred in the interior of the sleeve in spite of the impacts and vibrations to which the assembly can be subjected. A significant problem encountered in the use of such a device is the emission of gaseous components by the refractory material which, by condensing on the optical lens, can disturb the measurement. By judiciously selecting the composition of the refractory material and also controlling the atmosphere in the interior of the refractory tube, the influence of these gaseous components can be decreased. Another solution, proposed in the Patent Application WO-A2-03/029771, is the use of a second tube made of refractory material, which is impermeable to gases and is disposed in the interior of the cavity in the sleeve. The increased cost of this latter solution associated with a more complex device can easily be imagined, given that the thickness of the tube must not be greater than 5 mm in order to avoid an increase in the response time of the measuring system, and that the tube has to be positioned as close as possible to the cavity in the sleeve. However, the application does not give details with regard to the fixing means to be employed. The Patent EP-B1-1893959 likewise discloses a measuring device that uses an optical pyrometer comprising a sleeve made of refractory material that is dipped into the molten metal. This sleeve is centred by way of a guide tube and is supported on a fixed seat by way of articulated locking rods that are provided with wing nuts. The device then has to be placed in a cooled enclosure. The disadvantages of this device are numerous: specifically it requires that the sleeve made of refractory material is supported during the tightening of the nuts. Likewise, during the replacement of the sleeve, which takes place after a period of 15 to 24 hours of use—at the same time that the tundish is replaced—, it is not enough to loosen the wing nuts in order to pivot the rods so as to release the sleeve. Specifically, it is necessary to lift the sleeve beforehand from the seat on which it rests. Handling the hot refractory material is dangerous for the operator. In addition, a system of rods and wing nuts risks seizing given the extreme temperature conditions and the corrosive atmosphere. Finally, centring the measuring zone is also dependent on the positioning of the sleeve. The sleeve is centred by the guide tube of the measuring head which slides through the internal cavity of the sleeve and also by the positioning of the sleeve on the seat. The positioning thus depends on the external dimensions of the sleeve and the tolerances during its manufacture. Thus, when it is replaced, the measuring zone may move, with the consequence of less precise measurement.